1. Field of the Invention
The invention relates to a method of detecting and characterizing a segment of an artery by ultrasonic echography, using an array of ultrasonic transducers which produce a sectional plane or frame which is formed by L successive parallel excitation lines in a direction z which extends perpendicularly to the axis x of the artery, which array is associated with a transmitting circuit and a receiving circuit, said echography method being of the type involving signal processing in the time domain with correlation of the echographic signals relating to said lines and interpolation of the correlated signals in order to determine radial velocities v for points of said sectional plane which are limited to the traversing of the walls of the artery and its immediate vicinity.
In order to carry out the above method, the invention also relates to an apparatus for the detection and characterization of a segment of an artery, comprising an ultrasonic echograph and a work station, the echograph consisting of a probe provided with an array of ultrasonic transducers conceived to emit a frame of L successive lines during a period T in a direction z which extends perpendicularly to the axis x of the artery, a transmitting circuit and a receiving circuit, which echograph is of the type involving signal processing in the time domain for which purpose it comprises correlation means and interpolation means for the echographic signals.
2. Description of the Related Art
Methods and apparatus of this kind, suitable for use in the medical field, are particularly important for the characterization of a stenosis which may already have been detected either by angiography or by means of the ultrasonic probe itself.
An echograph is an apparatus for the examination of media, utilizing the ultrasonic radiation as an information source. An apparatus of this kind utilizes a step for the transmission of ultrasonic signals to the medium scanned by periodic excitations, as well as a step for the reception and processing of the echos returned by the obstacles encountered in the medium scanned. The two steps are executed by means of the same ultrasonic probe which is in contact with the medium. This probe is a construction which, generally speaking, is composed of a network of ultrasonic transducers which are assembled in the form of a linear array. It emits at a central frequency fo of the order of magnitude of a few MHz.
During the transmission step, the medium is selectively scanned along a line. In the receiving mode, the image of this scanned line is formed, taking into account the time of flight in the medium and the amplitude of the echos from the various obstacles encountered along the line. The image of a sectional plane is formed by the scanning of this line. In order to obtain a high image resolution, it is desirable to scan the medium very selectively by way of focused ultrasonic excitation and, in the receiving mode, to select the echos stemming from the same line by utilizing a focusing aperture.
A contemporary focusing technique consists of the use of a linear network of transducers and of defining, in the transmission mode, an incident beam which is focused by means of a delay rule imposed on the transducer excitation pulses. In the receiving mode, focusing is then achieved in a similar manner by suitably delaying the signals received by each of the transducers of the network, prior to their summing and later processing. This processing of the signals in the receiving mode, resulting in a signal of high amplitude for the echos stemming from the focal point (which point is situated on the line scanned) and in weak signals for all other echos is customarily referred to as a beam-forming method.
In order to achieve suitable focusing along an entire line, contemporary echographs utilize a focusing rule in the receiving mode which, generally speaking, is not continuous but subdivided into zones of the order of magnitude of a centimeter (cm) in the direction z of excitation. In the transmission mode, several beams which are focused to different depths are successively transmitted in the same direction. Consequently, it is to be noted hereinafter that when the part to be analyzed within an organism is small, for example of the order of magnitude of 2 by 3 cm, it may be that a single focusing zone suffices so that only one transmission is required, thus enabling the analysis frame frequency to be increased.
The use of linear networks of transducer elements not only enables focusing, but also the execution of the scanning necessary for the formation of a two-dimensional image on the screen of a conventional echographic monitor (2 D imaging). For the examination of, for example a segment of an artery of an order of magnitude of 3 cm, a frame of 128 lines which are spaced 0.25 mm apart may be used.
For non-invasive in vivo examination of an artery it is found that the detection and the evaluation of stenoses, both in respect of their origin and their seriousness, are an important goal for ultrasonic medical applications. Measurements utilizing the Doppler technique for the ultrasonic echos constitute the prime means of examination utilized by radiologists for this purpose. However, it .appears that the medical diagnosis results from criteria which are not very objective, such as the percentage of the reduction of the useful lumen area of the artery, or of the pressure gradient estimated on the basis of the Doppler measurement of the speed of the blood in the stenosis. Even though these methods are clinically renowned, it seems that they are only makeshift means which are not very accurate.
After detection of a stenosis, the physician must decide whether surgical intervention is necessary or a simple angioplasty (local dilatation) suffices. The latter is performed if the stenosis reduces the useful lumen area of the artery by not more than 70%, whereas the first solution is chosen when the vessel is almost occluded. In parallel with the morphologic detections by ultrasound or angiography, the velocity of the blood is the preferred parameter measured either by Doppler systems or by CVI (Colour Velocity Imaging) systems as described notably in the annual review L.E.P (Laboratoires d'Electronique Philips) 1990, in the article "De l'algorithme au produit", pp. 43, 44, by O. Bonnefous.
The simplified Bernoulli equation V.sup.2 =.DELTA.P (where V and P are the blood velocity and pressure, respectively) is supposed to provide an indication of the pressure gradient across the stenosis. However, this characterization is not specific enough, probably because the Bernoulli equation used does not correspond to the propagation phenomenon describing the relations between the pressure and the blood flow in the arterial system. Physicians are led to classify the stenoses in two groups, one group being qualified as significantly hemodynamic and the other as non-significantly hemodynamic, because the alteration of the vessel cannot be quantified. Moreover, an apparent contradiction exists between the fact that the blood velocity increases within the stenosis because of and in proportion to the reduction of the useful lumen area, and the fact that this velocity subsequently decreases abruptly when this useful lumen area decreases to the point where it causes a thrombus (artery nearly occluded). Hereinafter a more refined mathematical analysis will be developed, enabling this strange hemodynamic behaviour to be taken into account and forming a new approach to the characterization of stenoses. It will be demonstrated that the means proposed by the invention for the observation and the associated measurements in conformity with the in vivo acquisitions concerning normal and pathological cases are very well compatible with this new approach to the characterization of stenoses.
For the past ten years research has been carried in the field of ultrasonic echography in order to find out whether accurate characterization of the pulsating movement is possible, particularly of a given artery segment, especially to find out whether a part of the artery exhibiting a stricture (constriction, stenosis, atherosclerosis) can thus be characterized. In this respect reference can be made to French Patent Application FR-A-2 563 991 which proposes to search, for two excitation lines traversing a vessel, the boundaries of this vessel and the variation of these boundaries in the course of time, resulting in two dilatation curves which are slightly shifted in time, said shift being representative of the transmission time of the pressure wave, between the two excitation lines, within the vessel. For this manipulation use is made of a Doppler-type ultrasonic scanograph. However, such a principle for the measurement of echos lacks refinement and the means adopted to determine the position of the walls of the vessel are not very exact. On the other hand, it is not possible to construe more than two dilatation curves in the course of the same cardiac cycle; this is a source of irregularities during the rearrangement of the curves representing the analyzed segment of the vessel.